How to prepare WiFi for an on-stage demo?

Use a combination of proximity, directionality, less popular band, and increased transmit power:

• Put your wireless access point nearby your lectern or wherever your client device is going to be located, so you swamp other, more distant sources.
• Put a directional antenna on your AP, aimed toward your device and away from the preponderance of interfering devices. Also helps swamp the other sources. A directional antenna on your device will help too, if that's a possibility.
• Use -a or -n mode to limit the population of interfering and requesting devices.
• Use a firmware-modified AP that lets your boost its power (this last will help your AP's transmissions cut through the noise, but won't help your device's ability).

You could use a not-so-common 802.11 A router or 5 GHz 802.11 N. Each are on the 5 GHz frequency and the more common 802.11 b g n on 2.4 GHz will not cause dramatic interference.

Make the network closed so that it will not answer for connection requests without credentials.

The issue then becomes whether your devices support it. Most recent laptops do; few cell phones do; iPads do.

Wikipedia 802.11

It might be possible to use a directional Wi-Fi antenna to boost the reception within a limited area (i.e., the stage). And clearly the wireless network should be encrypted to help prevent malicious access. If there must be public Wi-Fi, then ideally the access points for this would be at the opposite end of the room, for what it's worth.

See also this related question at Server Fault.